CHAPTEE VII 



NEMERTEA 



I" Palaeonemertea — Oarinella, Polia. 

 iNEMEKTE A -j Schizonemertea — Li-neus, Gerebrottulus. 



(^ Hoplonemertea — Tetrastemma, Geonei/iertes, MalacoMella, 



CHAEACTEftiSTics. — TMs closs is characterised by a ciliated 

 ectoderm, which at the anterior end of the hody is sunk in, 

 and forms a pair of ciliated grooves or pits. There is a 

 protrusihle introvert, which m.ay he armed with hooks and 

 spines, opening above the Tnouth. A nerve commissure sur- 

 rounds it. The nervous system consists of two cerebral 

 ganglia, giving off two lateral nerves which extend through- 

 out the body, and may unite above the anus. The alimentary 

 canal is not branched, but may bear lateral caeca; it ter- 

 minates in an anus. The generative organs are simple 

 and paired ; the sexes are usually distinct. With few excep- 

 tions, the Nemertea are marine. 



The Nemertines are mostly found amongst seaweed and 

 coral rock, and they are frequently of the most brilliant colour. 

 Pelagonemertes is, however, pelagic, and like most pelagic 

 organisms is transparent, and Malacobdella is parasitic, living 

 in certain Lamellibranchs. A few inhabit fresh water, such 

 as Tetrastemma aquarum dulcium from Iforth America, and 

 an unknown species of the same genus recently found in 

 England. Two species, of Tetrastemma and two species of 

 Geonemertes are terrestrial. 



Probably the members of no class vary so much in size as 

 do the Nemertines ; many of them are quite small, whilst 

 others attain the length of many feet, and Professor M'Intosh 

 records finding a Zineus marinus which measured thirty yards, 

 and even then was only half uncoiled. They possess extreme 



